It is well known in the aluminum casting art that surface imperfections, such as pits, vertical folds and oxide formations, that form during ingot casting can develop into cracks during casting or subsequent processing steps. Material that exhibits cracks often requires expensive remedial rework to salvage the material, or outright scrapping of the material if the cracking is extensive. Most cast aluminum alloys are worked subsequent to casting, by various operations well known in the casting art to include: hot rolling, cold rolling, extruding, forging, drawing, ironing, aging, forming and stretching. However, working an alloy will not eliminate, or even reduce, surface cracks. One method of mechanically removing surface defects from an aluminum alloy ingot is scalping. Scalping is an expensive process that involves milling or machining off a surface layer along the surface of an ingot after it has solidified. If methods such as scalping are unsuccessful, the cracked ingot must be re-melted and re-cast, as defective ingots cannot be processed further and have little commercial value. Surface imperfections in aluminum cast ingots remain a significant problem in the alloy art.
In the past, control of surface defects has been accomplished by the intentional introduction of alloying elements or a protective blanketing gas to the casting process. However, many of the commonly used alloying elements added to limit surface defects have considerable drawbacks. Calcium, as well as sodium, can enhance edge cracking problems. Beryllium, another commonly added alloying element, has been banned from aluminum products used for food and beverage packaging. Blanketing gases, such as dichlorodifluoromethane, carbon dioxide, sulfur dioxide, sulfuryl fluoride, nitrogen trifluoride, sulfuryl chloride fluoride, sulfur oxifluoride and nitrosyl fluoride have been used as a means of limiting surface defects by varying the atmospheric conditions present during the casting process, however, the use of such gases poses health and environmental problems such as toxicity, breathing discomfort for personnel, residual sludge disposal concerns, corrosive atmosphere and green house gas global warming concerns.
There remains a need for an effective, inexpensive and clean alternative to the addition of alloying elements or blanketing gases having environmental concerns to prevent surface defects such as vertical folds, pits, oxide patches and the like from forming during the casing process.